A large number of printers have come into use in recent years and these printers are required to print at high speed and high resolution and with little noise. A printing apparatus that employs the ink-jet printing method (such an apparatus will be referred to as an “ink-jet printing apparatus” below) is an example of printing technology that meets these requirements. An ink-jet printing apparatus is capable of printing on a printing medium in non-contact fashion since it prints on the medium by discharging ink from nozzles provided on a printhead. As a result, a printing image can be formed stably on a wide variety of printing media.
Among these types of ink-jet printing apparatus, those that employ a method of printing by forming ink droplets using utilizing thermal energy are particularly simple in structure and therefore are advantageous in that the nozzles that discharge the ink can readily be packed close together at a high density.
In an ink-jet printing apparatus, however, stable discharge of the ink is required in order to perform printing by discharging ink from the printhead. In other words, it is required that the printhead of the ink-jet printing apparatus be durable and that it exhibit stable performance with respect to temperature fluctuation of the printhead and number of simultaneous discharges of the ink. Stable performance means that the amount of ink discharge, the discharge speed and the discharge precision (precision of the position at which ink is discharged) not vary according to conditions, such as a fluctuation in the temperature of the printhead.
Accordingly, in order to assure stable performance, printhead control in which driving pulses applied to the printhead are varied depending upon the temperature of the printer apparatus per se or the temperature of the printhead has been contemplated. In accordance with this conventional technique, the number of printing elements driven simultaneously varies depending upon the image to be printed and therefore the voltage supplied to the printing apparatus per se from the power supply also varies. As a consequence, there is a great change in voltage drop ascribable to the resistance of the wiring connecting the printing apparatus and the printhead. If a constant voltage is being impressed upon the printhead, the voltage applied to the printing elements within the printhead will differ for every image printed.
By way of example, in the case of an ordinary ink-jet printing apparatus, the wiring resistance between the printing apparatus per se and the printhead is on the order of 0.2 Ω, the head-contact resistance is on the order of 0.1 Ω and therefore the overall resistance is on the order of 0.3 Ω. If it is assumed that a driving current of 100 to 200 mA flows per printing element and that 54 printing elements are driven simultaneously, then the total current will be 5.4 to 10.8 A and the voltage drop due to the wiring will be 0.3 Ω×(5.4 to 10.8 A)=1.62 to 3.24 V. This is the voltage fluctuation to which the printing elements are subjected.
A fluctuation in the voltage impressed upon the printing elements leads to a fluctuation in discharge energy, namely a fluctuation in the discharge speed of the ink. Further, although the voltage impressed upon the printing elements provided in each of the nozzles of the printhead differs owing to simultaneous discharge of the ink, the driving voltage and driving pulses are decided in such a manner that the ink will be discharged stably when the number of simultaneous discharges is largest, i.e., when the driving voltage is greatest. When the number of simultaneous discharges is small, therefore, the printing elements are subjected to an excessively large driving voltage or driving pulses. This leads to a decline in the durability of the printhead.
In order to solve these problems, a thermal dot printing apparatus in which the driving pulse or driving time is changed in dependence upon the number of printing elements driven simultaneously has been proposed (e.g., see the specification of Japanese Patent Application Laid-Open No. 58-5280).
There has also been proposed an ink-jet printing apparatus in which an image signal transferred from a host device or the like is held temporarily in a buffer, the image signal is converted by an image processing circuit to a bit signal for every heating resistor within the ink-jet printhead, and the driving-pulse conditions are decided using a look-up table on the basis of the number of nozzles that discharge ink, the positions of these nozzles and temperature information obtained from a thermister provided in the ink-jet printhead (e.g., see the specification of Japanese Patent Application Laid-Open No. 9-11463).
According to yet another proposed ink-jet printing apparatus, the number of printhead nozzles to be driven simultaneously is counted before the printing of one scanning line, and a driving parameter is stored in a RAM and used based upon the value of the count (e.g., see the specification of Japanese Patent Application Laid-Open No. 9-11504).
In the examples of the prior art described above, however, the value of the voltage at the power-supply terminal of the printhead cannot be determined accurately merely by the voltage drop produced in accordance with the number of printing elements driven simultaneously. More specifically, if 56 printing elements are driven continuously as the maximum number of simultaneously driven elements, the driving voltage of the printhead will decline gradually. The amount of this voltage drop is not reflected in the driving pulse width decided by the number of simultaneously driven elements. The reason for this is that the supply capability of the power supply means that supplies the driving current to the printhead declines owing to continuous supply of large current.
FIGS. 4A and 4B are diagrams illustrating voltage drop due to number of printing elements driven simultaneously. A waveform 401 shown in FIG. 4A is a waveform of voltage fluctuation when 18 printing elements are driven simultaneously, and a waveform 402 shown in FIG. 4B is a waveform of voltage fluctuation when the maximum of 56 printing elements are driven simultaneously. A voltage drop VH_d2 indicated by waveform 402 is approximately three times larger than a voltage drop VH_d1 indicated by waveform 401.
FIGS. 5A and 5B are diagrams illustrating the states of voltage drop in an instance where the driving states shown in FIGS. 4A and 4B are allowed to continue. A waveform 501 shown in FIG. 5A illustrates the state of voltage fluctuation in a case where 18 simultaneously driven printing elements are driven continuously, and a waveform 502 shown in FIG. 5B illustrates the state of voltage fluctuation in a case where 56 simultaneously driven printing elements are driven continuously.
The waveform 501 shown in FIG. 5A indicates that the voltage drop remains at VH_d1 even upon elapse of a continuous driving time T1. On the other hand, in the case of waveform 502 in FIG. 5B, the amount of voltage drop increases with the passage of time when 56 simultaneously driven printing elements are driven continuously, with a voltage drop of VH_d3 being produced at continuous driving time T2 as the amount of fluctuation in voltage drop. As a result, the actual amount of voltage drop at time T2 is VH_d2+VH_d3.
Conceivable methods of preventing the occurrence of the fluctuation VH_d3 in voltage drop in continuous drive include (1) enlarging the capability of the power supply means that supplies the printing apparatus with power, and (2) providing large-capacity charge storing means between the power supply means and printhead to compensate for the fluctuation in voltage drop using the accumulated electric charge. However, both of these expedients raise cost and increase the size of the printing apparatus.